1. Field of the Invention
This invention relates to an apparatus for carrying out a process by plasma onto a substrate.
2. Description of the Related Art
Processes by plasma onto substrates are widely carried out in fields such as manufactures of semiconductor devices and liquid crystal displays. In forming a micro-circuit onto a substrate, for example, the plasma etching is carried out utilizing a resist pattern on the substrate as a mask. In preparing a conductor film or insulator film, the method of plasma-enhanced chemical vapor deposition (CVD), which utilizes a gas-phase reaction in plasma, has been practical. An apparatus for such a process generates plasma by an RF discharge or dc dipolar discharge in a process chamber, and carries out the process by function of the plasma onto a substrate placed in the process chamber.
Plasma processing apparatuses are required to increase plasma density for enhancing productivity by reducing process time. For increasing plasma density it is effective to confine plasma in a region, preventing the plasma from diffusing.
Employment of a discharge shield is a way to confine plasma. The discharge shield is a grounded metallic member surrounding a region where plasma is generated. Because no electric field is applied outside the discharge shield, the plasma is confined inside the shield. However, the discharge shield would bear the problem that the plasma-density distribution lacks uniformity because charged particles of the plasma flow into the shield, resulting in that plasma density decreases near the shield. In addition, if the current flowing to the ground via the shield does not contribute to the plasma generation for a process, it means loss of energy, i.e., decrease of electric power efficiency.
As a method to confine plasma without lack of plasma-density uniformity and loss of electric power, it is known to cover the surface of a shield exposed to plasma with an insulation layer. As well known, when a surface insulation layer covering a grounded metallic body is made contact onto plasma, the surface of the layer takes so called insulation potential, or sometimes called floating potential. Because of high mobility of electrons compared to ions, the insulation potential is negative in the range of several volts to several tens volts. Because essentially no charged particles flows into the surface being the insulation potential, there is no decrease of plasma-density and no loss of electric power.
However, it is impractical to completely surround plasma-generation region by a shield. In almost all cases, an opening is required at a shield. For example, an opening is required in case a plasma generation region is pumped, a gas is introduced to the plasma generation region, or a substrate is transferred to a position adjacent to the plasma. In such a case, even if the surface of a shield is made of insulator for confining the plasma inside, the plasma easily performs bipolar diffusion through the opening. Concretely, high-mobility electrons diffuse initially through the opening. Then, following them, ions diffuse to maintain electrical neutrality. As a result, the plasma diffuse to the outside through the opening. Therefore, plasma confinement without the plasma-density non-uniformity and the electric power loss is practically difficult.